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Got something to throw out there..
I’ll try to include as many details as I can

Myself (plumber) and a couple of our hvac guys installed 2 new RBI dominator boilers for a high school pool.
2 1/2” copper lines in and out...
The boilers and circ pumps and mixing valve were purchased from a local boiler supply house who sells the entire package together as a unit so I have to assume things are all specd to work together. We have purchased and installed roughy half a dozen of this set up with no issues..
Pump is B&G with 3” flanges and volute. I don’t know why it’s 3” when boiler inlet and outlet is 2 1/2 but..? Male adapters and Reducers on either side of the flanges.

1st time plumbed it all in using Apollo crimp fittings. All good. Approx 2 months and pinhole leak from just below the pump.
(Pump pulls water and feeds into boiler inlet)
Said what the hell...? Maybe a bad piece of copper. Cut it out and noted it was quite pitted but only in certain areas..
Replaced the section but sweat this time just to remove the crimp fittings from the equation.

Maybe 2 months and pinhole in the exact same spot. Cut it out and the same thing....
I’ve seen plenty of copper eaten away from much too powerful circ pumps. And that on domestic lines. This is obviously chlorinated pool water but we have 4 other pools and nothing like this.
Again this is one of 2 identical boilers side by side and the other has experienced no problems

Toss some ideas out.. I’m sure I missed a few things but I’ll try to answer any questions

Just the one clean line...no pitting anywhere else

Pump sits atop the flange in pic and pulls water down and into boiler...
line of pitting runs between my two fingers..

Turbulence?

Too many fitting too close together?

Preferential corrosion.

Cavitation.....its a biotch. Can your 90 be any farther away?

Elaborate big Jim...

That’s my general suspicion and have seen it elsewhere but never this extreme...??
What’s odd is this boiler has an exact twin right next to it with no problems..? Same pool(water chemical concentration) same pump same boiler and plumbed identically

Looks like iron in the water, copper and who knows if a water treatment chemical. Have been causing electrolysis, in a cavitation eddy! This happens behind pipe joints and elbows most often! Extra grounding may be in order!

Electrolysis

Slum the line comes straight down and into the pump so the only directional fitting in play is the lone 90

Are you operating on a performace spec or prescriptive spec?

e.g. - are you building off a set of design plans that say install this . . . or are you just told install something that works?

Haha! Beats me.


We were pulling a string of tubing out of an oil well one time.


Right along the seam....on one joint....was a line of pitting just like that.


Boss man said it was preferential corrosion.

Could be a difference in casting or the upstream flow characteristics....possibly inaccuracies in the frequency drive....just possibilities.

This.

BP...

Not ruling anything out.. care to elaborate? I understand dissimilar metal corrosion and that chlorinated pool water does not help that equation, but this seems awfully extreme and isolated?

MM..more so the latter. To be honest it’s new territory and a few of us just do the installs to learn. The company we buy the system from comes out and does the initial start up in accordance with warranty.. and they always pass, and in fact compliment the install..

Did you put a meter on it to see if there's any stray current?

Typically if we could stay 10ish pipe diameters before the fittings it smoothed out....5 was the minimum but often not enough to stop cav.